1. Field of the Disclosed Embodiments
This disclosure relates to systems and methods for reading ferroelectric memories and particularly systems and methods that employ high frequency alternating current (AC) signals for wirelessly and remotely reading ferroelectric memories avoiding a need for a conventional charge or sense amplifier as part of a memory cell to support the memory readout.
2. Related Art
Ferroelectric memories are non-volatile electric memory components that store information as remnant polarization in a ferroelectric material. A wide variety of usable ferroelectric materials exist. Often, the ferroelectric materials in ferroelectric memories are provided in the form of ferroelectric polymers including, for example, poly(vinylidenefluoride-co-trifluoroethylene) or P(VDF-TrFE), which tend to be very attractive in many ferroelectric memory applications based on the ease with which they can be physically manipulated and the ease with which the ferroelectric properties can be modified. Devices employing ferroelectric memories tend to have comparatively lower power usage and faster write performance than those using other memory technologies. These devices tend to support a greater maximum number of write-erase cycles. Also, these devices can be formed as printed circuits. These advantages are balanced against certain disadvantages including lower storage densities, storage capacity limitations, and higher costs.
Typically, the ferroelectric capacitor constitutes an electronic device in which the ferroelectric material is sandwiched between two electrodes to form the capacitor, with the ferroelectric material as the dielectric. In a simple, straightforward and conventionally-employed configuration, the ferroelectric capacitor will be in a parallel plate configuration, but other varied structures are possible and are often implemented.
Ferroelectric materials are characterized by having remnant polarization after an electric field has been applied and removed. A ferroelectric material has a nonlinear relationship between the applied electric field and the apparent stored charge. Specifically, the ferroelectric characteristic has the form of a hysteresis loop, which is very similar in shape to the hysteresis loop of ferromagnetic materials. FIG. 1 shows the typical hysteresis 100 of the polarization when a positive and negative electric field is applied across a ferroelectric capacitor. Hysteresis loops associated with ferroelectric materials show that typically when a positive or negative electric field is applied across ferroelectric materials such as in a ferroelectric capacitor, a particular polarization response results. If the applied electric field is of a sufficient magnitude, the capacitor will retain its polarization even after the field is removed. The ferroelectric capacitor is bistable, with two different polarizations possible when no electric field is applied. These can be used to represent the values “1” (see element 110 in FIG. 1) and “0” (see element 120 in FIG. 1). These points 110,120 are the two stable points at no electric field representing the values “1” and “0” of a 1-bit memory element for a ferroelectric memory. For a more detailed discussion, see Naber et al., “Organic nonvolatile memory devices based on ferroelectricity,” Adv. Mater. 22, 2010, pp. 933-45 (hereinafter “Naber”), which is incorporated herein by reference describing the state of the art in ferroelectric memories in organic nonvolatile memory devices.
A bit of data is written to the ferroelectric memory by applying a bias across the ferroelectric material. A positive bias may write one state (“1”) value and a negative bias may write another state (“0”) value, or vice versa depending on a polarization of a ferroelectric memory. When an external electric field is applied across a dielectric, the dipoles tend to align themselves with the field direction, produced by small shifts in the positions of atoms and shifts in the distributions of electronic charge in the crystalline structure. After the charge is removed, the dipoles retain their polarization state. The binary values of “0” and “1” are thus stored as one of two possible electric polarizations in each ferroelectrically-based data storage cell.
In a typical configuration, data is stored according to a binary polarization state of the ferroelectric capacitor, as described above. Writing to the cells is typically accomplished by (1) applying a positive bias across the ferroelectric capacitor to write a “1”, or (2) applying a negative bias across the ferroelectric capacitor to write a “0”. Reading from the cells is typically accomplished by applying a negative bias across the ferroelectric capacitor and measuring the amount of charge released by the capacitor. This charge may be measured using one of a sense amplifier or a charge integrator, either of which may be used to convert the charge into a large voltage. The amount of charge measured depends on the polarization state held by the ferroelectric capacitor, with a larger charge magnitude corresponding to a “1” state and a smaller charge magnitude corresponding to a “0” state. It is important to note that the above description refers to the cell having “held” a charge, because the reading process is destructive. After the reading process, the cell typically always holds a “0” value. Once the cell has been read, if the cell held a “1,” the cell must be re-charged to that value again. Also, those of skill in the art recognize that, as used in the above discussion, the designations of “positive,” “negative,” “1,” and “0,” and their relationships to one another are arbitrarily assigned and that other combinations are appropriate.